The present application relates to electronic lighting. More specifically, it relates to a dimming interface for a power line and will be described with particular reference thereto. It is to be appreciated that the present interface can also be used in other lighting applications and/or other power line applications, and is not limited to the aforementioned application.
In the past, dimmable ballast systems have typically been composed of multiple discrete ballasts. In order to achieve a lower light output, one or more of the ballasts would be shut off. Conversely, when greater light output is desired, more ballasts are activated. This approach has the drawback of only being able to produce discrete levels of light output. With each ballast only able to produce a single light output, the aggregate output is limited to what the various combinations of the ballasts present can produce. Moreover, this setup also requires multiple lamps for the same space to be lighted, resulting in an inefficient use of space.
Another approach in dimmable lighting applications has been to dim a single ballast by varying the operating voltage of the ballast, that is, by varying the voltage of the high frequency signal used to power the lamp. One drawback in such a system is that as the voltage of the high frequency signal is diminished, the lamp cathodes cool down. This can lead to the lamp extinguishing, and unnecessary damage to the cathodes. To avoid this problem, such systems apply an external cathode heating. While this solves the problem of premature extinguishing, the ballast is drawing power that is not being used to power the lamp. This decreases the overall efficiency of the ballast.
Another option is to reduce the range from full light output to a lower light output, but not low enough that external cathode heating is required. In T8 lamps, this amounts to a ballast that can change the lamp current from a high ballast factor level (typically 265 mA of arc current) to a low ballast factor level of only 140 mA. This provides a dimming range where a considerable amount of energy can be saved without sacrificing too much light. Associated with this high-low ballast factor approach is the interface between the power line and the ballast control input, which determines the light level. Conventional dimming interfaces have 2 output levels: a high ballast factor at which full power is output, and a low ballast factor at which less than full power is output. A drawback of conventional dimming interfaces is that they are subject to capacitive loading by non-dimming ballasts coupled to the circuit, which can cause the dimming interface to malfunction.
The following description provides new systems and methods that overcome the above referenced problem caused by capacitive loading by other dimming or non-dimming ballasts.